The biosynthetic mechanism of prostaglandin D2 in human platelet-rich plasma has been investigated. Platelet-rich plasma was separated into washed platelets and platelet-poor plasma, and [1-14C]prostaglandin H2 was incubated with each fraction. The enzymatic conversion of the endoperoxide to prostaglandin D2 was found only in platelet-poor plasma and not in washed platelets or platelet lysate. This prostaglandin D synthetase activity was purified to homogeneity and identified as serum albumin by sodium dodecyl sulfate polyacrylamide gel electrophoresis, isoelectric focusing, and immunoelectrophoresis. The optimal pH and Km value for prostaglandin H2 were 9.0 and 6 microM, respectively. Glutathione was not required for the activity. Although prostaglandin H2 ws converted to prostaglandin D2 and E2 in the reaction, only the prostaglandin D2 formation was dependent on the protein amount and abolished by prior boiling. The action of this activity under physiological conditions was examined in a model system constituted of serum albumin and washed platelets. Prostaglandin D2 formation was observed in association with thrombin-evoked platelet aggregation in this system and was proportional to the number of platelets and the concentration of serum albumin, suggesting that thrombin-stimulated platelets released prostaglandin H2, and the latter compound was then converted to prostaglandin D2 by the action of serum albumin. Consistent with this interpretation, prostaglandin H2 added to platelet-rich plasma was converted in part to prostaglandin D2, and the aggregation caused by this endoperoxide was greatly enhanced by neutralizing the action of prostaglandin D2 with anti-prostaglandin D2 antiserum.